The present invention relates to a reflection mask for use in lithography process for semiconductor integrated circuits, particularly a reflection mask for X-rays.
In projection lithography of transferring circuit patterns written on a mask onto a wafer, a high resolving power is required. For improving the resolving power, it is necessary to either enlarge the numerical aperture (NA) of a projection optical system or shorten the exposure wavelength. At present, a resolving power of 0.3 .mu.m or so is attained using a lens system having an NA of about 0.5 and an ultraviolet radiation of 248 nm in wavelength. A larger value of NA is not practical because the depth of focus of the projection optical system will be deteriorated. Now, projection lithography using X-ray in place of ultraviolet radiation is considered to be promising. In the X-ray region, the refractive indices of every materials are extremely close to 1, so it is necessary to use a reflection optical system. In order to obtain a high reflectivity, the reflection optical system is constituted by multi-layer mirrors.
Masks used in X-ray projection lithography are classified into transmission type and reflection type. A transmission mask comprises a membrane of a light element material which transmits X-rays and patterns formed on the membrane using a heavy element material which absorbs X-rays. Since the X-ray absorption coefficient is extremely large also in light element materials, it is necessary that the membrane thickness be extremely small, not larger than 1 .mu.m. Thus, the transmission type mask involves problems such as deterioration of the pattern positioning accuracy caused by stress and inconvenience of handling.
On the other hand, as described in Japanese Patent application Laid-Open No. 64-4021, there has been proposed a reflection mask comprising a multi-layer plane mirror and patterns formed thereon. FIG. 7 shows a construction of a conventional reflection mask. On a sufficiently thick substrate 1 there is formed a multi-layer 2 by laminating two kinds of materials with different refractive indices alternately in a large number of layers. This multi-layer exhibits a high reflectivity in the X-ray region. Part at normal incidence of the multi-layer is removed according to patterns, and the substrate serves as a non-reflective portion for X-rays. FIG. 8 shows another construction of a conventional reflection mask. A multi-layer 2 is formed on a substrate 1, for use as an X-ray reflecting portion. Part of the multi-layer is covered with an X-ray absorbing portion 8 according to patterns to form a non-reflective portion. The reflection mask, because of a thick substrate, is advantageous in that the distortion of pattern is difficult to occur and the handling of the mask is easy.